Metal Release from Manganese Nodules in Anoxic Seawater and Implications for Deep-Sea Mining Dewatering Operations

The potential mining of deep-sea polymetallic nodules has been gaining increasing attention due to their enrichment in metals essential for a low-carbon future. To date, there have been few scientific studies concerning the geochemical consequences of dewatered mining waste discharge into the pelagic water column, which can inform best practices in future mining operations. Here, we report the results of laboratory incubation experiments that simulate mining discharge into anoxic waters such as those that overlie potential mining sites in the North Pacific Ocean. We find that manganese nodules are reductively dissolved, with an apparent activation energy of 42.8 kJ mol–1, leading to the release of associated metals in the order manganese > nickel > copper > cobalt > cadmium > lead. The composition of trace metals released during the incubation allows us to estimate a likely trace metal budget from the simulated dewatering waste plume. These estimates suggest that released cobalt and copper are the most enriched trace metals within the plume, up to ∼15 times more elevated than the background seawater. High copper concentrations can be toxic to marine organisms. Future work on metal toxicity to mesopelagic communities could help us better understand the ecological effects of these fluxes of trace metals.


Supporting Information Text
Changes of nitrate+nitrite with time during the incubation.In all groups, nitrate+nitrite (NO3+NO2 or NOx) concentrations generally decreased with time (Fig. S3).The extent of NOx decrease was strongly influenced by the addition of acetate, but also slightly by temperature and the presence of Mn oxides.With no added acetate, NOx decreased slightly in the first 8 days of incubation in both G2 or G1 (+Mn-Ac±T), with the higher temperature in G2 leading to a more pronounced NOx decrease of about 5 μM between Days 8 and 74.
The trend of NOx in groups with 500 μM added acetate (G3-G6) all displayed an abrupt decrease of ~20 μM NOx after Day 1 and remained essentially zero.Concentrations of NOx reached 0 at Day 8 in G4 (+Mn+Ac+T), whereas NOx was completely depleted earlier at Day 4 in the other three groups (G3: +Mn+Ac-T; G5: -Mn+Ac-T; G6: -Mn+Ac+T) (Fig. S3b-c).Since there was no dependence on temperature in the absence of MnO2 (Fig. S3c), we attribute the slower drawdown of NOx in G4 (+Mn+Ac+T) (Fig. S3b) to the competing influence of Mn oxides, which has similar Gibbs free energy as NOx to act as an electron acceptor in the anoxic oxidation of organic matter 1 .Additionally, despite the much faster decrease of NOx in the presence of acetate after Day 1, there was little decrease in NOx between Days 0 and 1 for all groups, suggesting that the reduction of NOx did not start until after Day 1.

Changes of dissolved zinc and iron with time during the incubation. Dissolved trace
metal data from the groups with added acetate but not MnO2 (G6 or G5: -Mn+Ac±T) served as a control for identifying potential trace metal contamination from glass bottles, sampling, and from the acetate addition (Fig. S4).For example, dissolved zinc (Zn) concentrations increased rapidly and were characterized by similar or even higher concentrations than dissolved Mn (Figure 2), which was likely caused by contamination from glass bottles.Likewise, contamination from glass bottles and/or stainless-steel needles used for sampling may explain the noisy dissolved iron (dFe) data.The low and similar magnitude of maximum dFe concentrations between groups with and without Mn nodules suggests that the reduction of Fe from nodules was not important in our incubation experiment, which is consistent with the absence of active Fe(III) reduction observed within the oxygen deficient zones (ODZs) in the Pacific Ocean [2][3][4] .
Estimating Mn oxide reduction rates in ODZs during the U.S. GEOTRACES GP16 cruise.Surface Mn oxide concentrations, estimated as non-lithogenic particulate Mn, reach a maximum of 0.4 nM at 45 m and a minimum of 1.8×10 -3 nM at 235 m at Station 1 (12.0˚S,79.2˚W) during the U.S. GEOTRACES GP16 cruise in the Peruvian ODZ 5 .At Station 4 (12.0˚S,77.8˚W), concentrations of Mn oxides decrease from 0.15 nM at 20 m to 4.0×10 -4 nM at 300 m.We estimate the time scale of Mn reduction as the time for a Mn oxide particle to sink between the depths of the maximum and minimum Mn oxide concentrations according to Stokes' Law 6 .Assuming particle density as pure Mn oxides of 3.0 g cm -3 , seawater density of 1.027 g cm -3 , viscosity as 1.25×10 -3 kg m -1 s -1 , and size of Mn oxide particles of 1-2 μm 7 , we estimate a Stokes' sinking velocity of 0.1-0.Estimating residence time for crushed Mn nodules in the ODZ within the CCZ.The ambient plume is characterized by low particle concentrations and not expected to be influenced by strong particle aggregation 9 .Crushed Mn nodules in the waste plume have been estimated to have a median size of 9-12 μm 9 , too small to be easily recovered but still larger and thus faster sinking than the 1-2 μm size of naturally-occurring particulate Mn (pMn) 7 .Assuming a median size of 9-12 μm 9 , a particle density of pure Mn oxides of 3.0 g cm -3 , seawater density of 1.027 g cm -3 , and viscosity of 1.37×10 -3 kg m -1 s -1 for 11 ˚C seawater, we calculate a sinking velocity using Stokes' Law 6 of about 5.5-9.8m d - 1 for MnO2 particles of this size in the waste plume.Such sinking rates are very similar to the 0.1 mm s -1 (8.6 m d -1 ) previously calculated by Muñoz-Royo et al. 9 .It would take about 62 to 109 days to sink through the entirety of a 600 m-thick ODZ region.
Sensitivity tests of the estimated trace metal budgets.The trace metal budgets calculated in this study likely fall on the higher end of the metal budgets of a waste plume due to turbulent mixing during the ambient plume advection.The laterally transported ambient plume would be further diluted beyond the initial dilution factor of 100-1,000 at the dynamic plume stage.Muñoz-Royo et al. 9 modeled the extent of the plume for 90 days and used dilution factors of 40,000-400,000, demonstrating further dilution of more than 40 times during the transport of the ambient plume.This level of dilution would lead to similar or even lower concentrations of all measured trace metals (Mn, Cd, Co, Cu, and Ni) within the ambient plume than the background seawater concentrations.
Trace metal release rates and budgets are also sensitive to factors such as different dMn accumulation rates, and the concentrations and compositions of the sediment plume.
If a potential lower (0.07 nM day -1 ) or upper (368.6 nM day -1 ) bound of dMn accumulation rate within the waste plume was used, dissolved trace metal accumulation rates could correspondingly decrease by 82.9 times or increase by 63.6 times.The upper bound scenario leads to concentrations of all trace metals (Mn, Cd, Co, Cu, and Ni) far exceeding their background concentrations within the ODZ, whereas even Cu, Co, and Ni accumulation rates would be smaller than the background level in the lower bound scenario.Additionally, if sediment concentrations of the waste plume increased from 8 to 32 g L -1 , all trace metal accumulation rates would increase by a factor of four.In comparison, the trace metal budget is less sensitive to the choice of the apparent activation energy used in the calculation.If we use activation energies of 17.4 and 60 kJ mol -1 , the lower and upper bounds reported in the literature [10][11][12][13] , dMn accumulation rates at 11 ˚C become 8.7 and 4.4 nM day -1 , respectively.This is equivalent to a relative change of 49.3% and -23.8% as compared to 5.8 nM day -1 , which is much smaller than other sources of uncertainties.G3 and G4 (+Mn+Ac) 4.3±0.6×10 - 2.5±0.2×10 - 3.6±0.4×10 - 7.2±0.5×10 - Bulk Mn nodules 1.7×10 -5 8.5×10 -3 3.0×10 -2 4.2×10 -2 3 m d - 1 , leading to a reduction time scale at Station 1 as 640-2,559 days and at Station 4 as 943-3,771 days.The pseudo-first-order rate constant of Mn reduction at Stations 1 and 4 within the ODZ during the GP16 cruise is 1.6×10 -3 -8.4×10 -3 day -1 assuming excess of DOC concentrations (typical of 50 μM at ~200 m at Stations 1 and 4) 8 .

Fig. S1 .
Fig. S1.Schematic of the experimental setup.Seawater at each time point was sampled

Table S1 .
Summary of six groups of the incubation experiment

Table S2 .
Summary of ratios between trace metals and Mn (unit: mol:mol) in different